According to the National Flag Foundation and the Flag Research Center, when the US flag is displayed on a shor staff or on a floor mounted staff where it cannot be lowered to half staff, a proper way to indicate mourning is by tying a black bow made of ribbon above a full-staffed US flag.

By order of Congress, the five days that the US flag should be displayed at half-staff are:

May 15, Peace Officer's Memorial Day

Memorial Day, the last Monday in May. Only till noon, then the flag is raised to full staff.